1. Field of the Invention
The present invention relates to an image forming system in which a plurality of printers are connected to one another in a sheet conveying direction.
2. Description of the Related Art
Generally, when a digital multifunction device is used as a printer, data is sent, starting from the first page, and, for example, pages of a five-page document are printed in the order of the first, second, third, fourth, and fifth pages. In order to output printed pages in correct order on a paper output tray, sheets are discharged with their image forming surfaces facing down. Therefore, an image forming apparatus has a sheet reversing mechanism for reversing or turning over a sheet.
Examples of the sheet reversing mechanism include a sheet reversing mechanism of a switch-back type in which, after a sheet is pulled back into a reverse conveying path that branches from a straight discharge conveying path, the conveying direction of the sheet is changed and the sheet is delivered in the changed conveying direction. In this case, a sheet interval corresponding to at least the distance required for switchback is needed to ensure that no collision of successively conveyed sheets occurs in the reverse conveying path. Accordingly, control is performed so that the speed at which sheets are conveyed from a fixing device and discharged from the main body of the image forming apparatus through the reverse conveying path is higher than that for straight discharge in order to increase productivity without increasing the sheet interval. This control allows the discharge interval for reverse discharge to be made equal to the discharge interval for straight discharge (see, for example, Japanese Patent Laid-Open No. 6-161185).
When the discharge speed is changed, a sheet post-processing apparatus that receives a discharged sheet also needs to change the conveyance speed in accordance with the discharge speed from the main body of the image forming apparatus.
For example, if the discharge speed from the image forming apparatus is higher than the speed at which the sheet post-processing apparatus receives a discharged sheet, sheets may be jammed between the sheet post-processing apparatus and the image forming apparatus, thus suppressing normal conveyance. It is therefore desirable that the sheet post-processing apparatus and the main body of the image forming apparatus be controlled so that the conveyance speed provided by the sheet post-processing apparatus and the discharge speed from the main body of the image forming apparatus are substantially equal.
A tandem image forming system is also known in which two printers are connected to each other to achieve high-efficiency two-sided (or duplex) image formation (see U.S. Pat. No. 7,024,152). The tandem image forming system provides duplex copying by forming an image on one side of a sheet using a first printer, reversing the sheet, and then forming an image on the other side of the sheet using a second printer. The time required for the above configuration is shorter than the time required for duplex copying with a single printer.
If the conveyance speed of a sheet received by the post-processing apparatus from the image forming apparatus is different from the conveyance speed of the preceding sheet, the post-processing apparatus also needs to change the conveyance speed. The post-processing apparatus changes the conveyance speed of a sheet after no sheets remain in the conveying path in the post-processing apparatus or after the preceding sheet has passes through rollers driven by the same motor in a receiving unit in the post-processing apparatus. That is, it takes time for the post-processing apparatus to be allowed to change the conveyance speed. Therefore, a large sheet interval is provided before and after the sheet conveyance speed is changed, resulting in a reduction in productivity.
In a tandem image forming apparatus, there is no need to change the conveyance speed if printing is performed only in a duplex image forming mode. However, printing may also be performed in a one-sided (or simplex) image formation mode and a duplex image forming mode. When both a simplex image forming mode and a duplex image forming mode are used, the post-processing apparatus needs to change the conveyance speed. In this case, the benefits of increasing productivity due to duplex image formation being performed using a tandem image forming system are not sufficiently taken advantage of.